This invention relates to a system and method for the control of the density of extruded foam and more particularly to a system and method wherein control of an extrusion line is based, at least in part, on foam density as determined by comparing the volume of per unit time of extruded foam with the volume per unit time of unfoamed melt being delivered to an extruder die.
Currently, one of the fastest growing branches of the extrusion industry is the extrusion of plastic foam. There are many shapes of extruded product where a given bulk is desired, but strength requirements are modest, so that the foamed product has sufficient strength for the purpose. That is to say the strength of extruded solid plastic is not required. Typical examples are individual hamburger containers used at hamburger retail outlets. These familiar containers are, today, often formed in a "clam shell" design made from foam plastic sheet. The foam gives good thermal insulation and has as much strength as containers made for the same purpose from paper product. Foamed plastic, used in this way, provides a particularly good opportunity for cost savings. In order to save material when extruding solid plastic, only the dimensions can be changed. In other words, the amount of plastic used in the production of a particular product is dependent upon the cross-sectional dimensions of the extrusion. By keeping the oversize tolerance of such products to a minimum the amount of plastic consumed can be controlled, but even under the tightest extrusion control little savings are accomplished, because oversized cross-sectional dimensions are likely to be, in practice, only minimally above the upper dimensional limits.
With foamed plastic, material can still be saved by maintaining tight cross-sectional tolerances, but there is a much larger possibility, and that is controlling the density of the foam. If a given product is actually made with foam whose density is 70% of what it would be if it were made with a solid plastic, and that foam is compared with a foam that is only 35% as dense as the solid plastic, there is a 2:1 ratio in the amount of plastic which will be used in the two cases. And so it will be seen, that for foam plastic products extremely large variations in the amount of the plastic being consumed is possible even when the extrusion conforms to tightly held dimensional tolerances.
In the past, it has been suggested that a nuclear gauge, which measures mass per unit area, plus an independent dimension-measuring system which measures thickness directly, be used to determine plastic foam density. For foamed plastic sheet, for example, if the width and the speed of the extrusion remain constant and the actual thickness is known, then the volume per unit time of the foam being extruded is available. If the mass per unit time is determined, for example by a nuclear gauge, density can be calculated. Once density can be determined on a continuous basis, then a closed loop control system can be employed to control those extruder operating characteristics that affect density.
The inventor has successfully tried this approach, but has found, while the closed loop control operates as expected, the weakness of this system lies in the nuclear gauge, which is not very precise in its measurement of mass per unit area. Thus, a system and method for controlling plastic foam density that accurately determines density or a density-related control value, for the purpose of precise extrusion line control to minimize density and thus minimize the rate at which raw plastic is consumed, would amount to a considerable advance. The current invention accomplishes just that.